The present invention relates generally to Radio Frequency Identification (RFID) Tags with bridge circuit assemblies and the methods of producing such tags.
The design of a typical RFID tag reflects its origin in the semiconductor and printed circuit board industries. Although functional, the design has a number of features that increase the cost of the finished article. In a resonant RFID tag, the electrical inductance of an antenna is connected in parallel with a capacitor such that the resonant frequency of the thus-formed circuit is tuned to a prescribed value. In more advanced forms, the circuit of the RFID tag may include an integrated circuit die electrically and mechanically bonded to the antenna on a substrate, wherein the voltage induced in/on the antenna by a reader signal provides power to operate the integrated circuit on the die.
The antenna typically includes a metal coil pattern on one side of a substrate, and metallization on the second side of the substrate to cross over the antenna, i.e., bring the outer connection of the multi-turn antenna coil back to the open unpatterned area in the center, where the die is typically located and bonded. Vias, i.e., electrical connections through the substrate, connect the first side metallization to the second side metallization. Typically one connection is made at the outer perimeter of the antenna to the second side metallization, and a second connection interior to the antenna coil brings the second side metallization in contact with the die bonding pad on the first (coil) side metallization.
The die is bonded between the antenna and second side metallization such that it completes the circuit between the ends of the antenna. The antenna center frequency is often tuned by laser-trimming the area of a capacitor plate formed between the first surface and second surface metallization.
Several problems exist with the current method of manufacturing RFID tags. For example, because metallization is formed on both the front and back sides of the substrate, alignment between the two sides is crucial. Aligning the two sides presents challenges that are difficult to overcome and are costly. In addition, where the front side to back side alignment is not accurate, fabrication yield can be reduced.
Further, while the antenna and cross-over metallization Design Rules may be relaxed with large features and wide tolerances, the die-attach region requires tight tolerances on physical dimensions to match the relatively small pads on the die. Therefore, the antenna Design Rules, e.g., line width, line form, pad size and placement, space between features, etc., must meet the stricter requirements for the die bonding area with its small bonding pad features, thereby increasing the cost for the entire construction because of the special requirements in the one small bonding area. Alternatively, the die can be made to be very largexe2x80x94and therefore expensivexe2x80x94to meet the Design Rules and tolerances of the much larger antenna.
In addition, the material requirements of the die bonding process constrain the antenna substrate choices to those materials that are compatible with the die bonding process. This has the effect of increasing the cost, because the antenna could be made on inexpensive, but xe2x80x9clow performancexe2x80x9d substrates, except that the die bonding process may require a substrate that can withstand a moderate amount of heat, pressure, and/or process chemistry.
The present invention provides radio frequency identification (RFID) tag devices with bridge circuit assemblies and methods for high-volume, low-cost production. The construction of the devices and methods of the present invention presents a number of advantages over the prior art. For example, the present invention reduces the complexity of the typical two-sided RFID tag with through-substrate via connections by providing a one-sided circuit design. This obviates the need for patterned through-substrate via connections, while also addressing front side to back side alignment issues.
Another advantage of the devices and methods of the present invention is that the resonant frequency of circuits formed on the RFID tag devices of the present invention may be tuned by severing selected connections to one or more tuning capacitor plates that form a part of the capacitor structure. Severing connections to the tuning capacitor plates changes the capacitance of the circuit which, in turn, changes the resonant frequency of the circuit.
This invention may also reduce the number of connections in the RFID tag. The reduced number of connections may improve initial reliability and manufacturing process yield. In addition, fewer connections may also limit the sites of potential failure due to long-term aging conditions.
The devices and methods of the invention can be used to manufacture RFID tag devices that do not include an integrated circuit die for use as Electronic Article Surveillance (EAS) devices. Such devices may be deactivated by methods known in the art, such as exposing the device to a high intensity electromagnetic field at the resonance frequency. The large voltage induced in the circuit on the device may drive a current through any conductive medium used to join the circuit at the connection pads that is large enough to destroy the required electrical interconnection. Alternatively, the large voltage induced in the circuit may cause a conductive channel to form in the dielectric layer of the capacitor, thus destroying or changing the capacitance of the circuit. After deactivation, the device will no longer significantly interact with the electromagnetic field at the operating frequency of the inquiring system. Conversely, if the device is not deactivated, it will interact with a sensing field to indicate that an article is being removed from a controlled area.
In some embodiments of the present invention, an integrated circuit die may be attached to a die connection site that forms a part of the circuit, thus forming an RFID tag device including additional functions or features, e.g., memory, etc. The die connection site may be located either on the antenna substrate or on the bridge circuit assembly. The die connection site may, in some embodiments, include die connection terminals that are made by separating an integrated die connection pad before attaching the die. If desired, the deactivation methods described above may also be used to deactivate RFID tag devices that incorporate an integrated circuit die.
When the die is located on the antenna substrate, higher density Design Rules, i.e., the form and size of allowable lines, features, spaces between adjacent features, may be required on the antenna to accommodate the small features on the die, but then the bridge can be as simple as a piece of metal foil or metallized film substrate with no further patterning. The advantage of this approach is simplicity of the bridge design, and the choice between die-on-bridge vs. die on antenna substrate can be determined by the overall product cost. This invention has the flexibility to allow the user to place the die where it will be least expensive for the total system cost and optimize the design based on the total system cost with a minimum of design constraints.
The circuit patterns may be formed on a continuous web that can be separated to provide a number of individual RFID tag devices. The circuit patterns could be complete prior to separation of the web, or the circuit patterns could be partially formed, separated from the web, and then completed. Alternatively, a die could be attached at the die connection site either before or after the web is separated into the individual RFID tag devices.
Further, the modular construction of an antenna substrate and separate bridge circuit allows sub-optimization of each subsystem independently. For example, the antenna substrate may be fabricated using coarse Design Rules for high process yield, using inexpensive processes and materials. The separately constructed bridge can be fabricated using high density Design Rules, allowing the size of the attached RFID die to shrink significantly. The bridge can be fabricated using more tightly tolerancedxe2x80x94and therefore more expensive-manufacturing, but the bridge circuit is a small fraction of the total area of the completed RFID tag, and the absolute cost of the bridge will be relatively low. The cost of making a small bridge with high density Design Rules may be lower than the cost of making the entire antenna using high density Design Rules. For example, the substrates and metallization for each element, bridge and antenna, can be independently optimized to reduce cost while providing acceptable throughput and/or reliability.
The bridge circuit assembly also offers an easy way to make a tuning capacitor by supplying one of the two plates needed to form the capacitor. The antenna substrate provides the second capacitor plate. The plurality of tuning capacitor plates allows selective tuning by excision of one or more capacitor plate connections to optimize the electrical performance of the device.
In one aspect, the present invention provides a method for fabricating a radio frequency identification tag device by providing a base substrate including first and second major surfaces; providing a circuit pattern on the first major surface of the base substrate including an antenna pattern including first and second ends and a plurality of coils, wherein the first end of the antenna pattern is located within an inner space defined by the plurality of coils and the second end of the antenna pattern is located outside of the plurality of coils, a first connection pad and a second connection pad, wherein the first connection pad is in electrical communication with the first end of the antenna pattern and the second connection pad is in electrical communication with the second end of the antenna pattern, and a plurality of tuning capacitor plates, each of the tuning capacitor plates in electrical communication with the antenna pattern through a tuning capacitor plate connection; providing a bridge circuit assembly including a conductive layer; electrically connecting the first connection pad to the second connection pad through the conductive layer of the bridge circuit assembly; and forming a capacitor, the capacitor including the plurality of tuning capacitor plates and a common capacitor plate, wherein the conductive layer of the bridge circuit assembly forms the common capacitor plate.
In another aspect, the present invention provides a method for fabricating a radio frequency identification tag device by providing a radio frequency identification tag base including a base substrate including first and second major surfaces, and a circuit pattern on the first major surface of the base substrate including an antenna pattern including first and second ends and a plurality of coils, wherein the first end of the antenna pattern is located within an inner space defined by the plurality of coils and the second end of the antenna pattern is located outside of the plurality of coils, a first connection pad and a second connection pad, wherein the first connection pad is in electrical communication with the first end of the antenna pattern and the second connection pad is in electrical communication with the second end of the antenna pattern, and a plurality of tuning capacitor plates, each of the tuning capacitor plates in electrical communication with the antenna pattern through a tuning capacitor plate connection; providing a bridge circuit assembly, the bridge circuit assembly including a bridge substrate including first and second major surfaces, and a conductive layer on the first major surface of the bridge substrate; providing a dielectric layer between the radio frequency identification tag base and the bridge circuit assembly, wherein the conductive layer and plurality of tuning capacitor plates are separated by the dielectric layer; electrically connecting the first connection pad to the second connection pad through the conductive layer of the bridge circuit assembly; forming a capacitor, the capacitor including the plurality of tuning capacitor plates, the dielectric layer, and a common capacitor plate, wherein the conductive layer of the bridge circuit assembly forms the common capacitor plate; measuring a resonant frequency of the device; and selectively severing at least one of the tuning capacitor plate connections.
In another aspect, the present invention provides a radio frequency identification tag device including a radio frequency identification tag base including a base substrate including first and second major surfaces, an antenna pattern including first and second ends and a plurality of coils, wherein the first end of the antenna pattern is located within an inner space defined by the plurality of coils and the second end of the antenna pattern is located outside of the plurality of coils, a first connection pad and a second connection pad, wherein the first connection pad is in electrical communication with the first end of the antenna pattern and the second connection pad is in electrical communication with the second end of the antenna pattern, and a plurality of tuning capacitor plates, each of the tuning capacitor plates in electrical communication with the antenna pattern through a tuning capacitor plate connection; a bridge circuit assembly including a conductive layer; and a capacitor including the plurality of tuning capacitor plates and a common capacitor plate, wherein the conductive layer of the bridge circuit assembly forms the common capacitor plate, and further wherein the first connection pad is electrically connected to the second connection pad through the conductive layer of the bridge circuit assembly.
In another aspect, the present invention provides a radio frequency identification tag device including a radio frequency identification tag base including a base substrate including first and second major surfaces, an antenna pattern including first and second ends and a plurality of coils, wherein the first end of the antenna pattern is located within an inner space defined by the plurality of coils and the second end of the antenna pattern is located outside of the plurality of coils, a first connection pad and a second connection pad, wherein the first connection pad is in electrical communication with the first end of the antenna pattern and the second connection pad is in electrical communication with the second end of the antenna pattern, and a plurality of tuning capacitor plates, each of the tuning capacitor plates in electrical communication with the antenna pattern through a tuning capacitor plate connection; a bridge circuit assembly including a conductive layer; a capacitor including the plurality of tuning capacitor plates, a dielectric layer, and a common capacitor plate, wherein the conductive layer of the bridge circuit assembly forms the common capacitor plate, and further wherein the first connection pad is electrically connected to the second connection pad through the conductive layer of the bridge circuit assembly.
In another aspect, the present invention provides a method for fabricating a radio frequency identification tag device by providing a radio frequency identification tag base including a base substrate including first and second major surfaces, and a circuit pattern on the first major surface of the base substrate, the circuit pattern including an antenna pattern including first and second ends and a plurality of coils, wherein the first end of the antenna pattern is located within an inner space defined by the plurality of coils and the second end of the antenna pattern is located outside of the plurality of coils, a first connection pad and a second connection pad, wherein the first connection pad is in electrical communication with the first end of the antenna pattern and the second connection pad is in electrical communication with the second end of the antenna pattern, and further wherein the first connection pad and the second connection pad define a first axis that intersects both the first and second connection pads, and a plurality of tuning capacitor plates, each of the tuning capacitor plates in electrical communication with the antenna pattern through a tuning capacitor plate connection; providing a bridge circuit assembly having a longitudinal axis, the bridge circuit assembly including a bridge substrate including first and second major surfaces, and a conductive layer on the first major surface of the bridge substrate; providing a dielectric layer between the radio frequency identification tag base and the bridge circuit assembly, wherein the conductive layer and plurality of tuning capacitor plates are separated by the dielectric layer; electrically connecting the first connection pad to the second connection pad through the conductive layer of the bridge circuit assembly; forming a capacitor, the capacitor includes the plurality of tuning capacitor plates, the dielectric layer, and a common capacitor plate, wherein the conductive layer of the bridge circuit assembly forms the common capacitor plate, and further wherein each tuning capacitor plate of the plurality of tuning capacitor plates includes an opposed portion that is directly opposed by the common capacitor; and defining a capacitance for the capacitor by locating the bridge circuit assembly to selectively define the area of the opposed portions of each of the tuning capacitor plates.
In another aspect, the present invention provides a radio frequency identification tag device including a radio frequency identification tag base including a base substrate including first and second major surfaces, an antenna pattern including first and second ends and a plurality of coils, wherein the first end of the antenna pattern is located within an inner space defined by the plurality of coils and the second end of the antenna pattern is located outside of the plurality of coils, a first connection pad and a second connection pad, wherein the first connection pad is in electrical communication with the first end of the antenna pattern and the second connection pad is in electrical communication with the second end of the antenna pattern, and a plurality of tuning capacitor plates, each of the tuning capacitor plates in electrical communication with the antenna pattern through a tuning capacitor plate connection; a bridge circuit assembly including a conductive layer; and a capacitor having a capacitance, the capacitor including the plurality of tuning capacitor plates and a common capacitor plate, wherein the conductive layer of the bridge circuit assembly forms the common capacitor plate, wherein the first connection pad is electrically connected to the second connection pad through the conductive layer of the bridge circuit assembly, and further wherein at least one tuning capacitor plate of the plurality of tuning capacitor plates includes an opposed portion that is directly opposed by the common capacitor and an unopposed portion that is not directly opposed by the common capacitor.
These and other features and advantages of the devices and methods of the present invention may be discussed in more detail below in connection with various illustrative embodiments of the invention.